I have been extremely hazy on exactly how things would work if I bought something on one device and wanted access to it on other devices: specifically, if I bought an app on an iPad, would I have to syn both iPads to my MacBook to move it from one iPad to the other. One day, I decided I just couldn't be bothered, and attempted to rebuy an app (specifically, bubbles, altho I don't know why because my children are shockingly uninterested in it). R. had bought it through my account for T.'s iPad. When I attempted to rebuy it on my iPad for A., it gave me a prompt saying I'd already bought that one, did I want to re-download it.
Now _that_ is the way it should work.
I have not yet explored how the whole book and music thing works out.